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Financial Instruments (Details) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 CAD ($) MW | Dec. 31, 2020 USD ($) | Dec. 31, 2019 CAD ($) | Dec. 31, 2020 USD ($) MW | Dec. 31, 2019 USD ($) | Dec. 31, 2018 CAD ($)
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Risk management assets | $ 171 |  | $ 166 |  |  |
Decrease in base fair value | 14 |  |  |  |  |
Decrease in sensitivity values | $ 1 |  |  |  |  |
Fixed-price contracts | Minimum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Long-term fixed price power sale contract, delivery volume | MW | 300 |  |  | 300 |  |
Fixed-price contracts | Maximum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Long-term fixed price power sale contract, delivery volume | MW | 380 |  |  | 380 |  |
Structured products – Eastern US | Minimum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Basis relationship, percent |  |  | 91.00% |  |  |
Non-standard shape factors, percent |  |  | 63.00% |  |  |
Sensitivity analysis in derivative, basis relationship, percent |  |  | 4.00% |  |  |
Sensitivity analysis in derivative, non-standard shape factors, percent |  |  | 4.00% |  |  |
Structured products – Eastern US | Maximum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Basis relationship, percent |  |  | 112.00% |  |  |
Non-standard shape factors, percent |  |  | 116.00% |  |  |
Sensitivity analysis in derivative, basis relationship, percent |  |  | 6.00% |  |  |
Sensitivity analysis in derivative, non-standard shape factors, percent |  |  | 10.00% |  |  |
Long-term contracts | US | Fixed-price contracts | Minimum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Forward power price ranges per MWh |  |  |  | $ 24 | $ 20 |
Long-term contracts | US | Fixed-price contracts | Maximum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Forward power price ranges per MWh |  |  |  | 32 | 28 |
Level III | Long-term contracts | Fixed-price contracts |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Forward power price ranges per MWh |  |  |  |  | 1 |
Increase (decrease) in derivative forward power prices, reasonable possible |  | $ (1) |  |  |  |
Level III | Long-term contracts | Fixed-price contracts | Maximum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Forward power price ranges per MWh |  |  |  | 11 | 9 |
Level III | Long-term contracts | Forward contract |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Forward power price ranges per MWh |  |  |  |  | 6 |
Increase (decrease) in derivative forward power prices, reasonable possible |  | $ (6) |  |  |  |
Level III | Long-term contracts | Forward contract | Minimum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Forward power price ranges per MWh |  |  |  | 35 | 38 |
Level III | Long-term contracts | Forward contract | Maximum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Forward power price ranges per MWh |  |  |  | $ 52 | $ 60 |
Recurring fair value measurement | Level I |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Risk exposure associated with instruments sharing characteristic | $ (13) |  | $ (3) |  |  |
Recurring fair value measurement | Level III |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Risk exposure associated with instruments sharing characteristic | $ 582 |  | $ 686 |  |  | $ 695

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